Sound recording



. June 2,-1942. 'E. w. KELLOGG I 4 SOUND RECORDING Filed mamh 31., 1939 Patented Ju e 2, 1942 SOUND RECORDING.

Edward W. Kellogg Moore-stown, N. J., assignor to Radio Corporation of America, a corporation of Delaware ,Application March 31, 1939, Serial No. 265,081

18 Claims. '(Cl. Pill-100A) Thisinvention relates to sound recording and more particularly to a method of and means for recording hill and dale records with minimum distortion resulting from groove curvature and from the finite radius of the reproducing stylus tip.

In my Patent No. 2,122,207, I have disclosed and claimed a similar method and System by means of which the curvature of the recorded groove may be limited so as to -minimize distortion. According to that 'method, a compensating distortion is introduced intov the record groove by modifying the vertical movements of the recording stylus and, in order that the compensation may depend directly on the curvature of the wave to be recorded (distortion being a sating distortion may be effectively introduced.

by following the teachings of my above identified patent, the requirements change so much as the maximum groove curvature changes that only an approximation can be attained to the compensation which would be required to completely neutralize distortion due to the finite radius of the reproducing needle tip. The primary object of my present invention, therefore, is to provide an improved method of recording hill and dale records which will provide more precise compensation for the distortion resulting from groove curvature and the finite reproducing needie tip radius than heretofore.

More particularly, it is an object of my present invention to provide an improved method of and means for more accurately approximating the desired record groove shape to avoid the distortion heretofore noted.

Otherwise stated,'it is an object of my present invention to provide an improved method of and means for recording hill and dale sound records whereby the shape of the groove cut in a wax blank, 'for example, will be so modified that a reproducing needle of finite needle tip radius cooperating with said groove will execute movements corresponding substantially to the undistorted wave.

In accordance with my present invention, the cutting stylus, at the same time that it is executing a vertical movement, is displaced by an amount proportional to the vertical velocity it would have were no compensation being introduced. In this and subsequent statements, the term "proportiona is used in the broad sense of dependent upon and increasing with, but not necessarilyin the strict sense of maintaining a constant ratio. This is for the reason, as will be hereinafter explained, that the exact nature of the relationship between the displacement and the velocity is not thesame in' the several embodiments of the invention. The displacement may be either vertical and of double the frequency, ,or horizontal and of fundamental frequency, the latter being preferred. The displacement may be produced by an auxiliary magnetic element in the recorder, or by any other suitable means.

Another method of obtaining the desired compensation is to ITB-I'GCOId mechanically or electromechanically from a disc of identical dimensions,

itself, however, together with additional objects and advantages thereof, will best, be understood from the following description of several embodiments thereof, when read in connection with the accompanying drawing, in which Figure 1 is an enlarged fragmentary view of a reproducing stylus illustrating the-purpose of my present invention;

Figure 2 is a schematic view of a recordin system by means of which my present invention may be carried out in one manner;

Figure 3 is a schematic view of a recordin system by means of which my present invention may be carried out in another manner;

Figure dis a fragmentary view of a reproducing system employing the principle of my present invention;

Figures 5 and 7 are similar views showing how re-recording systems may be employed to derive the benefits of my present invention;

Figure 6 is a diagrammatic view illustrating a principle of this invention in connection with the systems of Figs. 5 and I;

Figure 8 is a view illustrating how my present invention may be availed of in re-recording from a photographic record to a mechanical record; and

Figure 9 shows a further system by means of which my present invention may be carried out in practice.

Referring more particularly to the drawing, I have shown, in Fig. l, the tip of a reproducing stylus i and a portion 2 of a record which is assumed to be moving in the direction of the arrow 1. The bottom of the record oove is circular in cross-section, the recording stylus being in the shape of a chisel, the forward or cutting face of which is V shaped but with the bottom rounded to approximately the same radius as that of the spherical reproducing stylus tip 5. In hill and dale recording, the recording stylus moves up and down when the record blank' 2 is horizontal, thereby cutting a groove of variable depth. In accordance with present practice, vertical movements are imparted to the recording stylus which correspond to the sound waves being recorded and, for minimum distortion, it would be desirable that the reproducing stylus execute vertical movements identical with those of the recording stylus. Since the groove is formed by a fiat cutting face, but reproduction is accomplished by a spherical reproducing tip, the movements of the latter will not be identlcalwith those previously executed by the recording stylus. The character of the resulting distortion, and of the compensation or correction required to avoid this distortion'may best be understood by comparing the instantaneous position of the center of the circular portion of the recording stylus with the center of the spherical end of the reproducing point (Fig. 1), the radius Rh of the circle being substantially the same as that of the sphere. These centers will obviously be the same distance above the bottom of the groove provided the groove bottom is horizontal, as indicated by the line EH. Since the cutting edge of the recording stylus is in a vertical plane through the point 0, it is obvious that the bottom of the groove will always be below this center bythe distance Rn. Thus, if the center of the recording stylus moves in the direction indicated by the line 0G relative to the record 2, it will cut a surface EA parallel to the line 0G and below the line 0G by the distance Rn. It isevident, however, from Fig. 1 that if the groove bottom is at the surface EA, the center 'of the reproducing stylus could not be at the point 0. The reproducing stylus would be displaced upwardly and this displacement means that it is no longer following the desired movements. In order for the center 0 to occupy its correct position, or, in other words, in order for it to follow the line 0G, the bottom of the record groove would have to be displaced downwardly to the line .FB. The point of contact would then be at C and not at F; that is, not directly below the point 0. Thus, in order that the reproducing stylus may followthe desired path, the recording stylus, instead of following a path which exactly represents the waves to be recorded, must be made tn follow a modified path wherein it is displaced during its upward and downward movements in such a manner as to cut a surface which is below that which an uncompensated groove would have. In other words, when cutting a horizontal portion of the roove, the center 0 of the recording stylus is sated recording, but when either an upward or downward slope is being cut; the recording stylus is displaced so as to cut the groove slightly deeper than it otherwise would. It will beevident that this lowered surface FB in Fig. 1 could be cut by lowering the recording stylus by the amount EF, or, lternatively, by displacing it longitudinally of he groove by the distance ED. In the disclosure f the present invention, I show several methods f providing either of these alternative compen ating displacements.

The arrangern nt shown in Fig. 2 of the drawing accompanying. the present specification accomplishes the same purpose and involves a slightly different principle from that involved in the invention described and claimed in my above identified Patent No. 2,122,207 in that it operates directly in terms of the functions of vertical velocity and displacement of the cutting stylus rather than acceleration.

Referring again to Fig. 1, if the cutting edge of a recording stylus were displaced from the point E to the point F, it would then cut the desired surface FB. To accomplish this, I have shown, in Fig. 2, a magnetic recorder 5 the armature I of which is coupled to a cutting stylus 3 by appropriate mechanical linkage such that the recorder 5 will provide the primary, or normal, ve tical movements of the cutting stylus 3, namely, a vertical velocity of the cutting stylus 3 equal to that which it is desired to impart to the reproducing stylus. The desired mechanical characteristics of the recording element 5 are usually obtained by connecting a mechanical resistance or damping device In to the armature 1. Included in the system of Fig. 2 is an electromagnetic unit 9 which provides secondary movement of the cutting-stylus 3, or the compensating displacement therefor, the stylus 3being also suitably coupled to the armature of the unit 9 by appropriate mechanical linkage such that the movements of the stylus 3 are a compound of those produced by the primary device 5 and the secondary device 9. The signal energy is supplied to the unit 9 after rectification by a full wave rectifier II and is controlled in magnitude by a proportionality adjusting device I3.

If the compensating unit 9 were inactive, the record cut by the stylus 3 under the influence of the recording element 5 would be, exactly like those conventionally made. The moving armature of the unit 5.may be controlled by meof the armature of the unit 9, with any attached elements, must be very high as compared with the frequency of the waves to be recorded, so

not displaced from the position whichit would 1 normally occupy in the case of an uncompenthat movements of its armature are controlled by elasticity of the armature thereof, and whatever load it carries must be very light and stiffly supported, with only so much damping as is required to prevent trouble from resonance. Un-' der these conditions, the displacement of the armature will be proportional to the current through the winding of device 9, whereas in the case of the device 5, it is the velocity of movement which is proportional to the winding current. It will be noted that, with the currents or driving forces of the two units 5 and 9 in phase and with resistance control of the primary recording element 5, but elastic control of the secondary or compensating movement imparting device 9, vertical displacements of the recording stylus 3 produced by the unit 9 are proportional to the vertical velocity of the cutting stylus, or, to the slope of the groove bottom. Whether the slope is negative or positive, the stylus displacement must be downward. Therefore, the current supplied to the element 9 is rectified by the rectifier Ii. The current supplied to the rectifier H' 'is proportional (instantaneously) to that supplied to the recording device 5, but for proper compensation, the factor of proportionality must vary with the linear velocity of the record at the radius where the roove is being cut, and this proportionality factor may be progressively varied by means of the control device I3. It is, of course, to be understood that if found desirable an amplifier y be introduced between the units 5 and 9, in addition to the rectifying device II and the controlling device l3.

Referring again to Fig.1, it is evident that for the purpose of cutting the surface FB, instead of theuncompensated surface EA, the recording stylus 3 may be displaced from the point E to the point D, instead of from the point E to the point F. This is preferable to the method just previously described in that the point D is closer to the point of tangency C where the reproducing stylus will make contact with the surface FE, and also for the reason that no rectifier will be required. The ideal condition would, of course,

be to displace the cutting edge to the point C,

R, tan

One method of providing the required horizontal displacements of the recording stylus 3 isindicated in Fig. 3. Here, as in Fig. 2, the primary signal recordingelectromagnetic element 5, provided with a mechanical resisance element, shown schematically at H], and the secondary,

compensating recording element 9 are employed.

The damper I5 is also included inthe element 9 to prevent chatter at its natural frequency,

- which must be well above the frequencies to be recordedin order to preserve correct phase relations. zontal, secondary, or compensating, displacements is proportional to and in phase with the primary force producing the verticalvelocities, but the horizontal movements are controlled by elasticity, whereas the vertical movements are controlled ,by mechanical resistance, thus .pro-

ducing horizontal displacement of the cutting stylus 3 proportional to the vertical velocities thereof. Since the required horizontal displacements are a function of the actual slope of the groove, which depends not only upon the vertical velocity of the cutting stylus, but also upon the linear speed of the record at the point where the groove is being cut, the factor of proportionality between the current supplied to the element 9 and that supplied to the element 5 in Fig. 3 must be varied as the cutter approaches the center of the record, and it will also be different for 78 and 33 The applied force to produce the horir R. P. M. records. In connection with Fig. 2, I have shown a control element l3 which can be progressively changed-during the process of cutting a record. In Fig. 3, I have shown a specific form of control for the same purpose. If a variable attenuator is employed, such, as those in.

will, in general,..be desirable to introduce someamplification between the devices 5 and 9 for the reason that the latter is likely to require more electrical power for its operation than the re-' cording element 5. Therefore, in Fig. 3, I haveshown an arrangement including an amplifier tube l9 by which the current ratio can be altered'with the progress of the recording. A voltage proportional to the current through the coil of the recordingelement 5 is applied to an adjustable potentiometer H, by which the desired fraction of this voltage can be applied to the grid of the amplifier tube l9, which I have shown as a pentode. The plate impedance of the pentode 19 is so high compared to the impedance of the winding of electromagnetic element 9 that the current sent through the coil of the electromagnet will not be appreciably affected by variations in the impedance of the coil, but will be proportional to and in-phase with the voltage applied tothe grid of the tube 99. This constitutes one of the simplest and most dependable methods of altering the current ratio. Other methods of avoiding the detrimental effects of changing coil impedance are, of course, possible,

- such as potentiometers made up of elements havbackwardly when it is on the ascending slope and forwardly when it is descending. This is the natural tendency if the needle has a slight fiexibility in the direction parallel to the record movement. Fig. 4 shows a reproducing device designed to have this characteristic. The general construction is similar to that of many existing hill and dale pickups, parallel motion of the moving coil being insured by use of two flat supporting springs v2-l which are anchored in a support 22. The unique feature of the reimposed on this continuous motion, the effects of friction-on these deflections prac ically disappear. In other words. the device behaves as though the record were infinitely slippery.v

A plug of rubber or other suitable material 25 is shown placed against the reproducing needle 23 to damp any tendency thereof to oscillate. If the vertical pressure on the needle 3 is practically constant (high average pressure as compared with changes due to vertical vibratory forces) and the horizontal movements are resisted by spring ac tion (rather than mass or resistance), the horizontal displacements of the needle, tip will be proportional to the slope of the groove bottom at the point of contact. Working on the reproducing system instead of on the recording system has the advantage of simplicity in that an adjustment which is correct for one linear record speed is also correct for other record speeds, since the force tending to deflect the needle forwardly and backwardly is dependent on the actual slope of the groove bottom. On the other hand, the amount of deflection probably cannot be as accurately controlled by this system as by the system which introduces the compensation in the making of the record, and it is preferable to introduce complications into a recording system rather than into a reproducing system.

If the record is made by a re-recording process, there are several ways in which the desired compensation might be introduced. simplest of such methods would be to transfer the recording from the master record by driving the cutter directly from a reproducing stylus as shown in Fig. 5 wherein a reproducing stylus 35 is shown in engagement with a master record 33, the stylus 3| being supported by the fiat springs 27, and coupled to the recording stylus 3, as shown. If the radius of the end of the reproducing stylus 35 is the same as that of the reproducing point to be used in playing the final record, the wave, as cut, will have the correct shape. This assumes that the reproducing stylus 3! of the re-recording device is made to track satisfactorily,

The principle underlying the re-recording system of Fig. 5 may be better understood by referring to Fig. 6 of the drawing. If a spherical object 3! moves in contact with a curve X, the center thereof will trace a path indicated by the curve Y which is at a constant normal distance Rn from the curve X. Conversely, if a sphere E la of the same radius travels in contact with a surface having the shape of the curve Y, the center thereof will follow the curve X.

The curve X represents the shape of the groove bottom of the master record, whichis assumed to correspond in wave form to the original sound. The center of the spherical reproducing tip 3| of the re-recording device of Fig. 5 would move in accordance with the curve Y of Fig. 6, and since the edge of the recording stylus 3 is at a fixed distance below the center of the reproducing tip, the groove cut by it will have the shape of curve Y. When the record thus cut is played, using a reproducing stylus of the same tip radius Rn as that of the stylus 3| of Fig. 5 or M of Fig. 6, the center of the sperical tip, as illustrated by the circle Ma of Fig. 6, will follow the path X, which is the condition to be met in order to reproduce the original sound without distortion. Such a transfer of the record from the master to the new blank might be carried out at a reduced speed so that the inertia of the reproducing and recording devices would not cause faulty tracking.

Since it might not be possible to line up the two surfaces of the master record 33 and the recording blank l with the required accuracy for cutting a groove of constant depth, an arrangement such as shown in Fig. 7 might be employed. In this arrangement, two fairly large masses 35 secured One of the to the reproducing stylus 3i and the recording stylus 3 tend to remain a constant distance apart, so far as rapid vibrations are concerned, and they thereby force the cutting stylus 3 to follow the rapid movements of the reproducing tip 3i, but

they permit slow relative movements to accommodate changes in distance between the two record faces. the main carriage serves to hold the reproducing tip 3i in contact with the groove in the master record 33. Another spring 38 holds a guide ball 36 in contact with the uncut surface of the wax blank 2, the guide ball 36 sliding in contact with said surface and. controlling the depth of the cut through a spring 39. An adjustment device G0 is indicated to regulate the average depth of the groove. It might be desirable, in such a transfer system, to make the original record 33 of perhaps double the dimensions of the final record I and, by means of lever action, to reduce the amplitude of the vertical motion. At the same time, the master record 33 and the blank 5 would have to be independently rotated at their appropriate relative speeds and fed past the recording element at speeds proportional to their respective groove pitches.

Instead of re-recording by direct mechanical action, it would be possible to so design an electro-rnechanical system that it would cut a groove in the wax identical in form with that which would be obtained mechanically by the process just described. This action requires that the reproducing styli bear identical relations to the dimensions or" their respective records and that all of the record dimensions, including amplitude of out, be in fixed ratio. A very carefully perfected system of voltage generation, amplification, and

conversion back to mechanical movement would be required in order to preserve the wave shapes. The requirements for systems with negligible distortion are well known to those skilled in the art.

If re-recording is done from film to disc instead of from disc to disc, the compensating distortion may be introduced at the point of reproduction from the film instead of in applying it at the recording stylus. Imparting forward and backward movements to the recording stylus, as in the system of Fig. 3, involves the introduction of a phase shift between the original and the re-recorded waves. The change in relative phase may be broughtabout either by a longitudinal movement of the recording stylus or a movement of the scanning light beam in the reproducing machine, an advance of the light beam having the effect of causing a given movement of the recording stylus to take place earlier than it otherwise would, while a retardation (movement in the same direction as the record travel) of the reproducing light beam delays the movement of the cutting stylus.

As described above, in connection with'Fig. 3,

it is necessary to produce a forward or backplitude on the film corresponds to the slope of the record groove. If the compensation is introduced at the reproducing machine, the re- A. spring 3'! anchored in a support 32 of quired deflection of the light beam in the direction of film travel, as shown by the arrow II in v Fig. 8, is proportional to the ordinate ofthe recorded wave, measured from the axis of the wave of a photographic record 53. Ordinates a to the right of the axis 5| mean upward slopes in the record groove, while ordinates b to the left mean downward slopes. For uncompensated reproducetion, the reproducing light beam 55 would be perpendicular to the axis 5| of the sound track. The compensation is introduced by tipping the light beam 55 as indicated in Fig. 8.

Thus, the light reaches a given point on the wave curve in advance of the normal time, if that point is on the left of the axis 5i, but somewhat later than the normal time if the point in question is on the right of the axis 5]. Moreover, it is, obvious that the amount of the advance or delay of the point of intersection of the light beam 55 with the boundary of the wave is directly porportional to the ordinate to the wave boundary, which, in turn, as has already been pointed out, is proportional to the upward or downward slope to be out in the record. Correct polarity relations must, of course, be preserved.

hit

It is also essential that the re-recording system be so designed that the velocity of the cutter is truly proportional to the instantaneous amount of light passing through the film. This requires not only a system with a fiat response characteristic, but also that there should be no relative'phase shift of the various component frequencies. For this reason, it is desirable to limit the range of audio frequencies to be recorded by suitable filters in the original film recording system, and then to design the cutting mechanism so that it is capable of recording ire-- quencies considerably higher than any which are recorded. This procedure is contrary to that commonly followed in disc recording, wherein it is common to'so design the cutter that it acts as a low-pass mechanical filter. As in the system described in connection with Fig. 3, in which horizontal deflections of the cutting stylus were produced, it is necessary to continuously change the degree of the tilt of the slit 55 as thedisc recording proceeds outside to inside of the record. Someof the design problems (especially of in Figs. 5 and '7 there is aforward and backward movement-of the point of contact which produces a similar eifect. Thus, all of the embodiments of my present invention, except that derelation of permanent or polarizing flux to the vibratory or alternating flux can be varied to give the desired amount of non-linearity; a relatively weak polarization and strong alternating component giving greater departures from linearity. Since the sharp peaks of the waves are at the top, or where the groove is shallow, and

the steepest part of the force-current characteristic of the magnet is that part in which the A.-C. and then-C. magnetizing forces are adding, or, in other words, when the pull is strongest, the maximum pull must occur when the stylus is in its highest position. This relationshipcould be provided by designing the device so that the magnets tend to pull the stylus upward, and having the movements controlled by elasticity rather than by mass or mechanical resistance. In other words, the device would have a very high natural frequency. On the other hand, the required condition could also be provided by employing a mass-controlled device,

or one of low natural frequency, with the moving element pivoted so that the pull of the magnet tends to accelerate the stylus downward, This arrangement, which is the one illustrated in the drawing in Fig.'9, is to be preferred to the other arrangement just described, for the reason that the mass controlled unit will require greater input, or driving force, for high frequencies than for low frequencies, and this will result in greater asymmetry in the high frequency waves, which are the ones which need such asymmetry for the purpose of compensating distortion. Thus, in Fig. 9, a mass'controlled cutter is employed, the mass reactance predominating over any mechanical resistance or elastic resistance over at least that portion of the frequency spectrum which contributes most of the extreme curvatures. The mass (rather than resistance) control is necessary in this case because there is no other way of introducing the integrating effect which, as described in my above noted patent, is necessary between the distorting element and the cutter. A relatively long air gap must be used so that non-linearity will not be introduced by appreciable relative in the case of the mechanical copying illustrated scribed in connection with Fig. 2, canbe gener alized in the terms just stated.

Another method of producing the modification of the vertical accelerations of the cutter, and more nearly akin to-that disclosed in my above identified patent, consists in utilizing the nonlinear relation between current and pull of a magnet (the pull being proportional to the square of the flux densityin the airgap); The

changes in air gap length. Such non-linearity would be in the wrong direction and depends on deflection rather than acceleration. Since some, elastic element must be employed to maintain the air gap length and the average groove depth, resonance. might occur at some frequency within the audio-range. Therefore, I provide a damp-- ing element 51 which it is assumed also supplies some elastic restoring force.

Although I have shown and described several ways of carrying out my invention in practice, it will be apparent to those skilled in the art that many other ways and systems are possible. I, therefore, do not wish to be limited except insofar as is made necessary by the prior art and by the spiritof the appended claims.

I claim as my invention:

1. The signal recording method which comprises actuating a recording element in accordance with said signals to effect movement thereof corresponding to said signals, and simultaneously causing said element to be displaced from its normal movement by an amount proportional to the slope of the curve being recorded.

2. The method of recording signals on a .blank which comprises actuating a recording element in accordance with'said signals along a predetermined axis relative to said blank to eil'ect movement thereof along said axis corresponding element to be displaced along said axis from its normal position therein by an amount proportional to the slope of the curve being recorded.

4. The method of recording signals on a blank which comprises actuating a recording element in accordance with said signals along a predetermined axis relative to said blank to effect movement thereof along said axis corresponding to said signals, and simultaneously causing said element to be displaced in a direction transverse to said axis of motion by an amount proportional to the velocity of movement which said element would normally have along said-axis in response to said signals in the absence of said displacement.

5. The invention set forth in claim 3 characterized in that said displacement is of double the frequency of normal movement of said element along said axis.

6. fhe invention set forth in claim 4 characterized in that said displacement is of the same frequency as the normal movement of said element along said axis.

7. The method of recording on a blank a hill and dale record of signals which comprises actuating a recording element in accordance with said signals to produce movement thereof in a direction normal to the surface of said blank, and simultaneously causing said element to be displaced toward said blank from the position it would normally occupy during said movement by an amount proportional to the slope of the curve being recorded.

8. The method of recording on a continuously movable blank a hill and dale record of signals which comprises actuating a recording element in accordance with said signals to produce movement thereof in a direction normal to the surface of said blank, and simultaneously causing said element to be displaced in a direction parallel to the plane of continuous movement of said blank by an amount proportional to the velocity of said normal movement.

9. The method of recording on a blank a hill and dale record of signals which comprises actuating a recording element in accordance with said signals to produce movement thereof along an axis normal to the surface of said blank, and simultaneously causing said element to be displaced along said axis from the position it would at any instant occupy as a, result of said movement and by an amount proportional to the velocity of said movement and at a frequency which is double the frequency of normal movement of said element in said direction.

10. The method of recording on a blank a hill and dale record of signals which comprises actuating a recording element in accordance with said signals to produce movement thereof in a direction normal to the surface of said blank, and simultaneously causing said element to be displaced at right angles to said direction by an amount proportional to the velocity of said movement and at the same frequency as the frequency of movement of said element in said first named direction.

11. In apparatus for recording a signal track upon a blank, the combination of a recording element, means for actuating said element in accordance with the signals being recorded to produce primary movement thereof in a predetermined direction relative to said blank, and means for effecting displacement of said element as a function of the slope of the track formed during said primary movement.

12. In apparatus for recording a signal track upon a blank, the combination of a recording element, means for actuating said element in accordance with the signals being recorded to produce primary movement thereof in a direction normal to the surface of said blank, and means for effecting displacement of said element as a function of the slope of the track formed during said primary movement.

13. In apparatus for recording a signal track upon a blank, the combination of a recording element, means for actuating said element in accordance with the signals being recorded to produce primary movement thereof in a predetermined direction relative to said blank, and means for effecting displacement of said element in the same direction as a function of the slope of the track formed during said primary movement.

14. In apparatus for recording signals upon a blank, the combination of a recording element,

means for actuating said element in accordance with the signals being recorded to produce primary movement thereof in a predetermined direction relative to said blank, and means for effecting displacement of said element in a direction normal to said first named direction as a function of the velocity of said primary movement. I

15. The method of recording a hill and dale recordof signals upon a continuously moving blank by means of a recording element arranged for movement at right angles to' the surface of the blank at the recording point which comprises first making a variable width photographic record of said signals with the aid of a light beam extending across said record at right angles to a predetermined axis thereon, then deriving signal energy from said variable Width record with the aid of a reproducing light beam extending across said axis at some other angle thereto, and finally applying said derived signal energy to said recording element to actuate said element whereby to cause it to cut a groove of varying depth in said blank.

16. The invention set forth in claim 15 char-' aeterized by the additional step of causing said second named angle to vary in accordance with the linear velocity of said blank relative to said recording element.

17. In the method of translating by means of a translating device signals onto or from a record thereof wherein said signals are represented by a signal track of varying slope and wherein said record moves continuously relative to said translating device, the step which comprises effecting a corrective movement of said translating device relative to said record in a direction parallel to the direction of continuous relative movement between said record and said device, said corrective movement being at signal frequency and of a magnitude such as to produce displacements of said device relative to said record proportional to the slope of the curve of said track at the point where correction takes place. ,I t

18.' Inthe method of forming by means of a. cutting tool a hiil-and dale sound groove in a record blank, which groove is to be reproduced later by a reproducing stylus having a tip of finite size,the step of compensating for the distortion normally resulting from the finite size of said reproducing stylus which comprises advancing or delaying the time at which a. given portion of said groove is out during recording, the amount of advance or delay being a. function or the slope'ot the groove at the point where the advance or delay takes place, I

' EDWARD w. KELiDGG. 

